The present invention relates to a rotor for an electric motor having encapsulated permanent magnets. Such rotors are used particularly in brushless motors.
Sintered anisotropic magnets have good magnetic properties, but they are not easily manufactured as precise multipolar radially oriented cylinders. As a result, they are normally formed as segmental magnets which must be mounted on the rotor. If magnet segments such as this are glued on to the rotor, problems may arise as a result of centrifugal forces and imprecise air gaps.
As disclosed in published specification No. 63-144745 (A) of Japanese Patent Application No. 61-290466, it is known to secure permanent magnets on the rotor base of a brushless motor by embedding the magnets in a coating of plastic or resin material which also fills the circumferential spaces between the magnets. This provides a simple way of securely and precisely positioning the magnet segments on the rotor. However, this does not provide the optimal flux because the radial spacing between the magnet segments and the stator is increased and because the circumferential spaces between the magnet segments are also filled with the non-magnetic plastic or resin material.
If the plastic or resin material is loaded with a particulate high permeability, low reluctance ferromagnetic filler material, such as soft iron filings, to overcome this problem, the filler material merely provides a flux leakage path between adjacent magnets and reduces the radial magnetic fields provided by the magnets.